Dissecting cosmic-ray electron-positron data with Occam's Razor: the role of known Pulsars

TL;DR

The paper argues that nearby pulsars can naturally explain both the PAMELA positron fraction rise and the ATIC e± spectrum using benchmark e± production models and diffusion propagation. It demonstrates that a single pulsar generally cannot account for both datasets, but a small ensemble of nearby pulsars can plausibly fit PAMELA, while ATIC may require a subset of more powerful sources or multiple contributors. The authors employ the ATNF pulsar catalogue and explore diffusion setups, injection indices, and energy-output models, highlighting uncertainties in pulsar ages, distances, and velocities. They forecast that Fermi-LAT will critically test the pulsar scenario by providing precise spectra, unveiling new pulsars, and constraining anisotropies in high-energy lepton arrival directions.

Abstract

We argue that both the positron fraction measured by PAMELA and the peculiar spectral features reported in the total electron-positron (e+e-) flux measured by ATIC have a very natural explanation in electron-positron pairs produced by nearby pulsars. While this possibility was pointed out a long time ago, the greatly improved quality of current data potentially allow to reverse-engineer the problem: given the regions of pulsar parameter space favored by PAMELA and by ATIC, are there known pulsars that explain the data with reasonable assumptions on the injected e+e- pairs? In the context of simple benchmark models for estimating the e+e- output, we consider all known pulsars, as listed in the most complete available catalogue. We find that it is unlikely that a single pulsar be responsible for both the PAMELA e+ fraction anomaly and for the ATIC excess, although two single sources are in principle enough to explain both experimental results. The PAMELA excess e+ likely come from a set of mature pulsars (age ~ 10^6 yr), with a distance of 0.8-1 kpc, or from a single, younger and closer source like Geminga. The ATIC data require a larger (and less plausible) energy output, and favor an origin associated to powerful, more distant (1-2 kpc) and younger (age ~ 10^5$ yr) pulsars. We list several candidate pulsars that can individually or coherently contribute to explain the PAMELA and ATIC data. Although generally suppressed, we find that the contribution of pulsars more distant than 1-2 kpc could contribute for the ATIC excess. Finally, we stress the multi-faceted and decisive role that Fermi-LAT will play in the very near future by (1) providing an exquisite measurement of the e+e- flux, (2) unveiling the existence of as yet undetected pulsars, and (3) searching for anisotropies in the arrival direction of high-energy e+e-.

Figures (16)

• Figure 1: Scatter plot in the energy output in $e^\pm$, in ergs, versus age (left panels) and versus distance (right panels), for the ST model and for the CCY model. The black circles indicate all pulsars in the ATNF catalogue, while the red dots indicate "gamma-ray" pulsars ($g<1$).
• Figure 2: The ratio of the predicted $e^\pm$ energy output from "gamma-ray" ($g<1$) pulsars for the HR, ZC and CCY models over the naive prediction (ST model), as a function of age (left panel) and of distance (right panel).
• Figure 3: The effect of electron-positron escape from the Galaxy (defined by the suppression fraction ${\rm cyl}(\xi)$), as a function of the thickness of the galactic diffusion region $L$, for selected pairs of values for the injection time and energy.
• Figure 4: The spectrum of selected nearby pulsars and SNR's (for the parameters employed to calculate the fluxes see tab. \ref{['tab:nearby']}). We assume an $e^\pm$ injection spectral index $\alpha=2$, and a median diffusion setup (MED). The dotted lines correspond to injection spectra featuring an exponential cutoff at $E_{e^\pm}=10$ TeV.
• Figure 5: The positron fraction for the same sources as in fig. \ref{['fig:spectrum_nearby']} and tab. \ref{['tab:nearby']}.